Fluid flow control devices, such as a control valves and regulators, are commonly used to control characteristics of a fluid flowing through a pipe. A typical device includes a valve body defining an inlet, an outlet, and a fluid flow path extending between the inlet and the outlet. A valve seat ring is coupled to the valve body and defines an orifice through which the flow path travels. A throttling element, such as a plug, is moveable relative to the valve seat ring thereby to control fluid flow through the orifice.
Certain fluid flow control devices employ a cage-style trim in which a cage is provided for guiding movement of the throttling element. The cage defines an interior bore sized to receive the throttling element and includes at least one passage through which the fluid flow path passes. The throttling element is moveable to a closed position in which the throttling element closes off at least one passage through the cage. Because of machining tolerances, however, a thin annular gap is present between an exterior surface of the throttling element and the interior bore surface of the cage. This gap may allow fluid to flow through, thereby creating a potential leak source when the device is intended to be in the closed position. To fully close the device, a bottom edge of the throttling element is typically driven by a closing force supplied by an actuator into the valve seat ring, thereby to provide a primary seal in the fluid flow control device.
Conventional primary seals formed by throttling elements pressed against valve seat rings are prone to leaks. A primary leak path is formed in the clearance between throttling element and cage which extends from the cage passage to the valve seat ring orifice. Fluid pressure upstream of the primary seal creates a pressure differential across the seal. As a result, any imperfections in the mating surfaces or other disruptions of the seal will allow fluid to leak when the throttling element is in the closed position. Such leaks may erode the valve seat thereby accelerating the rate of leakage, which in turn exacerbates seat erosion.
The leakage and erosion problems are even more pronounced when the fluid flow control device is used in an erosive environment. In certain applications, such as valves used to control the flow of water into a boiler in a power plant, tend to erode the primary seal more quickly. Power plant applications have historically been fairly non-erosive when the plant was started only a few times each year and typically operated 24 hours a day. More recently, power plants are started on a daily basis and operate only during peak-load daytime hours. As a result, scale that has built up on the inside of water pipes tends to loosen and break off as the pipes expand and contract during heating up and cooling down periods each day. These loosened scale particles have a high hardness and can become entrained in the fluid flow as it passes through the pipe and any fluid flow control devices disposed therein. The velocity of water passing through the pipes used to supply the boilers is relatively high, and therefore scale particles entrained in the water impinge on the primary sealing surfaces and quickly erode the valve seat. Valve seat erosion prevents the valve from shutting off the water flow, reduces power plant efficiency, and causes further damage to the fluid flow control device.
One traditional approach to solving the erosion problem has been to use harder materials for both the seating and the throttling element. While this approach works for certain applications, many power plants have recently started using chemicals having corrosive properties to treat the boiler feed water. Frequent cycling operation also makes it more difficult to control water chemistry. In general, harder materials tend to be more susceptible to corrosion, and therefore this approach may be used only in limited applications.
Another known approach has been to use a soft meal seat on the seat ring with a hard metal seat on the throttling element. The throttling element is then pressed against the soft seat ring with sufficient force to make a new seat each time the throttling element closes. Again, this approach works for limited applications and suffers from several draw backs. First, anything trapped between the seating surfaces as the throttling element closes will prevent full shut off, resulting in high velocity fluid flow across the seat which quickly erodes the soft seat material. If the throttling element is somehow able to shut completely, the debris will create an indentation in the soft seat material. When the valve is subsequently opened and the debris is flushed away, the indentation will create a leak path in the seat which again results in high velocity fluid flow and erosion of the seat material when the throttling element is subsequently closed.